A great variety of surface vehicles are known and include conventional motor vehicles, toy vehicles and vehicles having special purposes, such as cleaning vehicles. Generally all such vehicles are retained on their respective support surfaces by gravity. Exceptions do exist, however such as trains which operate at steep angles of ascent and descent which are retained by specially designed tracks or cams and vehicles which travel on a curved track and are retained thereon by centrifugal forces.
Vehicles have been proposed which are retained on support surfaces by magnetic or electric fields. These have yet to become practical.